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Lenses for changing PST Focal Length

Posted: Mon May 07, 2018 8:58 am
by AndiesHandyHandies
Hi

What size and focal length singlet lenses do I need to adapt the PST to work in collimation mode with a F7.5 telescope?

Does the type of objective lens affect abberations? What sort of lens is the PTS objective, I assume a doublet as one was taken apart to polish off the failed ERF coating. Is a larger doublet ideal to use then?

I assume the collimator needs to be -150mm using the rule - Objective F No x 20 = -Nmm.

And where can they be obtained from? And can they be a drop in replacement for the existing lenses?

It would be useful to have a table of F No scopes and the sources of lenses for each F No.

Andrew.

Re: Lenses for changing PST Focal Length

Posted: Mon May 07, 2018 11:38 am
by AndiesHandyHandies
Hi

Just come across Marks post with an alternative method of reaching focus without having to chop off a telescope tube.

viewtopic.php?f=9&t=12866&p=128272&hili ... ns#p128272

Edmund Scientific stock negative achromats
https://www.edmundoptics.co.uk/optics/o ... ic-Lenses/

Only F5 and F7.5 at 25mm, F5, F6 and F7.5 at 40mm.

Andrew.

Re: Lenses for changing PST Focal Length

Posted: Wed May 09, 2018 10:36 pm
by AndiesHandyHandies
Hi

Are these simple lenses similar to those used in the PST Collimator?

https://america.optosigma.com/products/ ... oated.html

Andrew.

Re: Lenses for changing PST Focal Length

Posted: Mon May 14, 2018 5:19 am
by marktownley
yup. they're similar.

Re: Lenses for changing PST Focal Length

Posted: Mon May 14, 2018 9:25 am
by AndiesHandyHandies
Hi Mark.

If you change the collimator do you need to have a matching re-focuser to balance out the abberations?

Are they a drop in fit? Are they bigger or smaller if not.

Thanks. Andrew.

Re: Lenses for changing PST Focal Length

Posted: Mon May 14, 2018 12:45 pm
by marktownley
Re-focuser needs to be the same diameter and focal length (positive not negative) to balance out the aberrations.

Re: Lenses for changing PST Focal Length

Posted: Tue May 15, 2018 7:38 am
by AndiesHandyHandies
Hi Mark,

Thats what I thought.

Are the Optisigma lenses a drop in? Personally the Lunt 110mm at 95mm would give F10 on the Meade 127mm Triplet to try. I am in the middle of a town on a windy ridge, near 'Windy Gap', so seeing might be a limitation. The Vixen would be more convenient to take out to rural site.

Their -200mm is longer than the ES doublets on offer, and is the same? as PST.

We are going to try the PST on the Vixen 110L. We could move the collimator into the 1.25" T2 stub if we need a bit more focus travel.
Putting the Lunt 110mm on the front, fits in the dew cap, and Baader 1.25" 35nm as blocker in the T2 stub for now. If it works put in a Beloptic 1.25" KG3. We have KG3 modded eyepiece for testing. Try Pentax thread 2x and 3x tele-extenders on the back to magnify, as others have done.

Thanks. Andrew.